As quantitative analysis of Thallium-201 uptake and washout are increasingly used for the non-invasive detection of coronary artery disease, optimal definition of "normal limits" becomes imperative to maximize the sensitivity, specificity and predictive accuracy of this test. We and others have now pointed out that gender differences are present in thallium washout, suggesting that gender-matched normal data ranges may be important. In the next phase of this study, we will examine the physiology of this phenomenon in normal subjects, with serial analysis of whole body, blood, and serum thallium kinetics after exercise, as well as analysis of renal clearance and muscle mass. We will test the hypothesis that the relatively greater muscle mass in men acts as a "sink" for Thallium-201, affecting the analysis of washout from the myocardium.